Filters with virtual inductor implemented using negative capacitor

Abstract

A filter includes a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept the same current. The filter also includes an input terminal to accept an input voltage across the circuit and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor.

Claims

1. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current, wherein the negative capacitor is formed using a Ferroelectric Oxide (FEO) layer sandwiched between two metal layers, wherein the FEO layer includes one or combination of Barium titanate (BaTiO.sub.3), Strontium titanate (SrTiO3), Hafnium Zirconium Oxide (HfZrOx) and Doped Hafnium oxide; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor.

2. The filter of claim 1, wherein the resistor is formed by a transistor operating in a triode region.

3. The filter of claim 1, wherein a thickness of the FEO layer is less than a critical thickness T.sub.c determined based on
T.sub.c=1/(2C.sub.cap), wherein is a material based parameter of the material of the FEO layer, C.sub.cap is the capacitance of the positive capacitor.

4. The filter of claim 1, wherein a material of the FEO layer is doped.

5. The filter of claim 1, wherein the output voltage is taken across the positive capacitor, such that the filter is a passive low-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade.

6. The filter of claim 1, wherein the output voltage is taken across the resistor, such that the filter is a passive high-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade.

7. The filter of claim 1, wherein the circuit is a resonant circuit, wherein the output voltage is taken across the positive capacitor, and wherein, in a resonant frequency band, the circuit is a passive filter with the output voltage across the positive capacitor greater than the input voltage applied across the circuit.

8. The filter of claim 1, wherein the circuit is an integrated circuit formed on a substrate.

9. The filter of claim 8, wherein the circuit comprises: a dielectric oxide layer sandwiched between a first metal layer and a layer of the two metal layers, wherein the first metal layer extends beyond the dielectric layer, and wherein the extended portion of the first metal layer is patterned; and.

10. The filter of claim 9, wherein the first metal layer is arranged on the substrate.

11. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor, wherein the output voltage is taken across the positive capacitor, such that the filter is a passive low-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade.

12. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor, wherein the output voltage is taken across the resistor, such that the filter is a passive high-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade.

13. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor, wherein the circuit is a resonant circuit, wherein the output voltage is taken across the positive capacitor, and wherein, in a resonant frequency band, the circuit is a passive filter with the output voltage across the positive capacitor greater than the input voltage applied across the circuit.

14. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor, wherein the circuit is an integrated circuit formed on a substrate, the circuit comprises: a dielectric oxide layer sandwiched between a first metal layer and a second metal layer, wherein the first metal layer extends beyond the dielectric layer, and wherein the extended portion of the first metal layer is patterned; and a ferroelectric layer sandwiched between the second metal layer and a third metal layer.

15. A filter, comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor or the positive capacitor, wherein the resistor is formed by a transistor operating in a triode region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a conventional 1st order passive low pass filter.

(2) FIG. 2A shows an exemplar Gain vs Frequency curve for a first order passive low pass filter of FIG. 1.

(3) FIG. 2B shows an exemplar Gain vs Frequency curve for a 2nd order passive low pass filter formed by cascading two first order passive low pass filters of FIG. 1.

(4) FIG. 3A shows a block diagram of a filter according to some embodiments.

(5) FIG. 3B shows a circuit diagram of a circuit of a filter according to one embodiment.

(6) FIG. 4 shows a cross section of a negative capacitor according to some embodiments.

(7) FIG. 5 shows a schematic of voltages across a negative capacitor and an inductor used by some embodiments.

(8) FIG. 6 shows a schematic of a negative capacitor stabilized with help of a positive capacitor according to one embodiment.

(9) FIG. 7A shows a plot of the charge voltage characteristics of a positive capacitor formed according to some embodiments.

(10) FIG. 7B shows a plot of the charge voltage characteristics of a negative capacitor according to some embodiments.

(11) FIG. 8A shows a plot of the Energy vs Charge characteristics of a positive capacitor according to some embodiments.

(12) FIG. 8B shows a plot of the Energy vs Charge characteristics of a negative capacitor employed by some embodiments.

(13) FIG. 9A shows a plot of the energy landscape curve of a ferroelectric oxide material used by some embodiments.

(14) FIG. 9B shows a plot of polarization vs electric field used to calculate material based parameter a according to some embodiment of the invention.

(15) FIG. 10 shows a plot of Gain vs Frequency characteristics of a filter with virtual inductor according to some embodiments.

(16) FIG. 11 shows a 3-D schematic of the filter in semiconductor platform according to one embodiment.

(17) FIG. 12A shows a schematic of a circuit of a low pass according to one embodiment.

(18) FIG. 12B shows a schematic of a circuit of a high pass filter according to one embodiment.

(19) FIG. 12C shows a schematic of a circuit of a resonant circuit according to one embodiment

(20) FIG. 13 shows the block diagram of a method for fabricating a filter as a semiconductor device according to some embodiments.

DETAILED DESCRIPTION

(21) FIG. 3A shows a block diagram of a filter 300 according to some embodiments. The filter 300 includes a circuitry 330 including a resistor 301, a positive capacitor 303, and a negative capacitor 302 connected in series to accept the same current. The order of connection of the resistor and the positive and the negative capacitors can vary. The filter 300 also includes an input terminal 310 to accept an input voltage across the circuit and an output terminal 330 to deliver an output voltage taken across the resistor or the positive capacitor. In some embodiments, the filter 300 includes only passive elements, such as resistors and capacitors. In some embodiments, the filter 300 includes only a combination of the resistor, the positive capacitor, and the negative capacitor.

(22) FIG. 3B shows a circuit diagram of a circuit of a filter according to one embodiment. The circuit includes a resistor 301, a capacitor 303 and a negative 302 capacitor. As shown on this diagram, all these elements 301, 302, and 303 are combined is such a way that once connected to a voltage source, the same current 304 pass through all the elements; this particular electrical connection is known as series connection and referred herein accordingly.

(23) The power roll off the filter with a negative capacitor can be even higher than 40 dB/decade and the gain of the filter with a negative capacitor can be positive. The negative capacitor differs from the positive capacitor in that the charge associated with a positive capacitor increases with the increase of the voltage across the positive capacitor, while the charge associated with a negative capacitor decreases with the increase of the voltage across the negative capacitor.

(24) FIG. 4 shows a cross section of a negative capacitor 400 according to some embodiments. Those embodiments form the negative capacitor using a Ferroelectric Oxide (FEO) layer 430 sandwiched between two metal layers 410 and 420.

(25) Some embodiments are based on recognition that the voltage across the negative capacitor includes a term resembling the voltage across an inductor. After some simulations and/or experimentations, some embodiments confirm that the negative capacitor acts, in part, as an inductor. Hence, the negative capacitor can be potentially used in place of the inductor. Hence, the negative capacitor used in the circuits for the purpose of its inductive purposes is referred herein as virtual inductor.

(26) FIG. 5 shows a schematic of voltages across a negative capacitor and an inductor used by some embodiments. The voltage 510 across the negative capacitor is

(27) $\begin{array}{cc}{V}_{\mathrm{FE}}\left(t\right)={\mathrm{lt}}_{\mathrm{FE}}\frac{d^{2}Q}{{\mathrm{dt}}^2}+t_{\mathrm{FE}}\frac{\mathrm{dQ}}{\mathrm{dt}}+2t_{\mathrm{FE}}Q+4t_{\mathrm{FE}}{Q}^3+6t_{\mathrm{FE}}{Q}^5,& \left(1\right)\end{array}$
where Q is the total charge, t.sub.FE is the thickness of ferroelectric oxide forming the negative capacitor, l, , , and are material constants of FE oxide.

(28) The voltage 520 across an inductor is

(29) $\begin{array}{cc}{V}_L\left(t\right)=L\frac{d^{2}Q}{{\mathrm{dt}}^2},& \left(2\right)\end{array}$

(30) Some embodiments are based on realization that the first term 515 of Eqn. 1 is quite similar to the term 525 of Eqn. 2. To that end, it is realized that a negative capacitor has a built-in virtual inductance of value lt.sub.FE.

(31) Some embodiments are based on recognition that the negative capacitor is unstable in isolation, but can be stabilized if connected in series with a positive capacitor. Some embodiments are based on another realization that the positive capacitor plays its role in the filters to attenuate high-frequency signals. To that end, the same positive capacitor can play the dual role in the filter, i.e., to attenuate high-frequency signals and to stabilize the negative capacitor. In such a manner, the power roll off a filter with virtual inductor as a negative capacitor can be increased without the need to use the actual inductor.

(32) FIG. 6 shows a schematic of a negative capacitor 400 stabilized with help of a positive capacitor 610 according to one embodiment. In this embodiment, the positive capacitor 610 is formed using dielectric oxide layer 440 sandwiched between two metal layers 420 and 450.

(33) FIG. 7A shows a plot of the charge voltage characteristics of a positive capacitor formed by a layer 440 according to some embodiments. This plot demonstrates that the charge associated with a positive capacitor increases with the increase of the voltage across the positive capacitor.

(34) FIG. 7B shows a plot of the charge voltage characteristics of a negative capacitor formed by the FEO layer 430 according to some embodiments. This plot demonstrates that the charge associated with a negative capacitor goes down with the increase of the voltage across the negative capacitor.

(35) FIG. 8A shows a plot of the Energy vs Charge characteristics of a positive capacitor formed by a layer 440 according to some embodiments. This plot has a V shaped Energy vs Charge curve (U vs Q). The curvature of U vs Q curve gives the value of capacitance.

(36) FIG. 8B shows a plot of the Energy vs Charge characteristics of a negative capacitor employed by some embodiments. This plot has an inverted/upside down V shaped Energy vs Charge curve (U vs Q). The curvature of U vs Q curve gives the value of capacitance. However, such a negative capacitor is unstable without additional assistance and configuration.

(37) FIG. 9A shows a plot of the energy landscape curve 300 of a ferroelectric oxide material as a function of charge used in FEO layer 430 according to some embodiments. The energy landscape curve of a FEO material has W shape 900. This curve 900 around zero charge value has negative curvature giving rise to negative capacitance, referred herein as a negative capacitance zone. Normally a FEO material cannot stay in this zone because it has higher energy and end up being either of the two local minima 920 and 930. However, adding a capacitor 610 in series to have the same charge can make the ferroelectric oxide stable in negative capacitance zone. This is because adding a normal capacitor makes the overall energy of the system lower.

(38) To that end, some embodiments select the thickness of the FEO layer 430 as a function of charge of the positive capacitor. For example, one embodiment selects the thickness of the FEO layer to be less than a critical thickness T.sub.c determined based on
T.sub.c=1/(2C.sub.cap),
wherein is a material based parameter the material of the FEO layer, C.sub.cap is the capacitance of the positive capacitor.

(39) FIG. 9B shows the measured polarization vs electric field characteristics of ferroelectric oxide. The electric labeled in this figure as 960 is known as coercive electric field (E.sub.c) and the polarization at zero electric field labeled by 950 is known as remnant polarization P.sub.R. Once P.sub.R and E.sub.C are known from the P-E measurement, one can calculate a by using the following equation,

(40) $=-\frac{3}{4}\frac{E_C}{P_R}.$

(41) Some embodiments are based on recognition that typically it is impractical to connect two capacitors in series, because the joint capacitance of two positive capacitors connected in series is less than the sum of their individual capacitance. However, some embodiments are based on realization that However, some embodiments are based on realization that, when a negative capacitor is added in series with a positive capacitor the joint capacitance is enhanced. The amplification of the input voltage in the invented filter can be understood from Kirchhoff s Voltage Law (KVL), according to this law,
V.sub.inputV.sub.RV.sub.outputV.sub.FE=0;

(42) Here, V.sub.R is the voltage across the resistor and V.sub.FE is the voltage across the negative capacitor. In most applications V.sub.R0 and

(43) $V_{\mathrm{FE}}=-\frac{\mathrm{Charge}}{.Math.C_{\mathrm{FE}}.Math.};$
using these values we get,

(44) $V_{\mathrm{output}}=V_{\mathrm{input}}+\frac{\mathrm{Charge}}{.Math.C_{\mathrm{FE}}.Math.}.$

(45) Therefore, V.sub.output>V.sub.input, since for a positive applied voltage charge is positive.

(46) In such a manner, the gain of a filter with virtual inductor as a negative capacitor can be positive without the need to use any active elements, such as transistors and operational amplifier.

(47) FIG. 10 shows a plot of Gain vs Frequency characteristics a filter with virtual inductor according to some embodiments. Here Gain is defined as 20 log(H(f)) wherein H(f)=V.sub.out(f)/V.sub.in(f). As is evident from the figure, the value of Gain for the filter in the pass band 1010 is +3 dB which is greater than 0 dB. And power roll off that is the slope of Gain curve in the stop band beyond cutoff frequency 1020 is greater than 40 dB/decade.

(48) A frequency of interest can be defined as a critical frequency fc at which the Gain is 0 db. The said critical frequency can be thought of a frequency beyond which the negative capacitance effect is nullified. As shown in FIG. 10 the gain G() is a function almost constant at the low frequency regime and decreases sharply at the higher frequency. An expression for gain is

(49) $\mathrm{Gain}\left(\right)=20\log \left(\frac{1}{C_{\mathrm{Cap}}{t}_{\mathrm{FE}}\sqrt{{}^2+{\left(\frac{2}{}+\frac{1}{t_{\mathrm{FE}}{C}_{\mathrm{Cap}}}-\frac{{}^{2}l}{}\right)}^2}}\right).$

(50) An expression that relates fc with the circuit parameters and the material based parameter of FEO is

(51) $f_C=\frac{1}{2}\sqrt{\frac{-A+\sqrt{A^2-4B}}{2}},\mathrm{where}$$A=\frac{{\left(t_{\mathrm{fe}}\right)}^2}{{\left(t_{\mathrm{fe}}l\right)}^2}-\frac{4}{l}-\frac{2}{{\mathrm{lC}}_{\mathrm{Cap}}{t}_{\mathrm{fe}}}$$\mathrm{and}$$B=\frac{4{}^2}{l^2}+\frac{4}{l^2{C}_{\mathrm{Cap}}{t}_{\mathrm{fe}}}.$

(52) As long as the required the cutoff frequency of the filter is less than that of the critical frequency, the negative capacitance can provide positive Gain.

(53) A second order filter can be implemented by having an inductor in series with the first order filter, however having an inductor in an integrated circuit is expensive as mentioned earlier. In addition to power roll of and gains benefits of the filter with the virtual inductor implemented via negative capacitor, such a filter according to various embodiments is more compact than the filter with actual inductor and can be advantageously implemented as an is an integrated circuit formed on a substrate.

(54) FIG. 11 shows a 3-D schematic of the filter in semiconductor platform according to one embodiment. The filter includes a negative capacitor which is formed by a ferroelectric oxide layer 1105 sandwiched between two metal layers 1104, 1106; a positive capacitor formed by a dielectric layer 1103 sandwiched between two metal layers 1102 and 1104; and a resistor formed by pattering the extended portion labeled 1110 of the metal 1102. Here, the negative capacitor, positive capacitor and the resistor are in series combination. For example, the dielectric layer 1103 can include one or combination of Al2O3, SixOy, SixNy, SixOyNz, Teflon, HfO2, or any other dielectric with a dielectric constant below 200. For example, the substrate can include one or combination of silicon (Si), silicon carbide (SiC), diamond, and gallium nitride (GaN).

(55) Depending on arrangement of the output terminal, the filter of some embodiments can for a low-pass filter, a high-pass filter, and a band-pass filter that can be also configured to form a resonant circuit.

(56) FIG. 12A shows a schematic of a circuit 1230 of a low pass according to one embodiment. In this embodiment, the filter built with a series combination of a resistor 1201, a capacitor 1203 and a negative capacitor 1202 wherein the input terminal 1210 is taken across the whole series combination and output terminal 1220 is taken across the positive capacitor 1203. Such combination of input and output terminals would provide low pass filter operation, which presents less attenuation to low-frequency signals than high-frequency signals. Specifically, with the output voltage taken across the positive capacitor, such that the filter is a passive low-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade.

(57) FIG. 12B shows a schematic a circuit 1240 of a high pass filter according to one embodiment. In this embodiment, the filter built with a series combination of a resistor 1201, a capacitor 1203, and a negative capacitor 1202, where the input terminal 1210 is across the whole circuit and the output terminal 1225 is across the resistor. Specifically, with the output voltage is taken across the resistor, such that the filter is a passive high-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20 dB per decade. In some implementations, this circuit behaves like a high pass filter and is capable of providing gain in the pass band and would yield more than 40 dB/decade power roll off.

(58) FIG. 12C shows a schematic of a circuit 1250 of a resonant circuit according to one embodiment. In this embodiment, the filter built with a series combination of a resistor 1201, a capacitor 1203, and a negative capacitor 1202, where the input terminal 1210 is across the whole circuit and the output terminal 1226 is across the capacitor and the negative capacitor. When the circuit operates at the resonant frequency the reactance due to the virtual inductor coining from the negative capacitor cancels out the reactance of the positive capacitor and a high current flows through the circuit, in this way a series resonance is achieved.

(59) FIG. 13 shows the block diagram of a method for fabricating a semiconductor device according to some embodiments. The method includes providing substrate 1310, here the substrate includes but not limited to silicon (Si), silicon carbide (SiC), diamond, gallium nitride (GaN) and so on. Further the method includes the formation 1320 of a first metal layer for the resistor and positive capacitor. The formation of this metal layer can be done by Lithography.fwdarw.Metal Deposition.fwdarw.Lift-off and/or Metal deposition.fwdarw.Lithography.fwdarw.Etching. Here the lithography could be performed using, including but not limited to photo-lithography, electron-beam lithography. Metal deposition can be done using one or combination of an ebeam deposition, a joule evaporation, a chemical vapor deposition and a sputtering process.

(60) The method also includes 1330, deposition of dielectric layer on the first metal layer to form positive capacitor. Then etch away the dielectric layer from the extended region of the first metal layer so that an electrical contact can be made while doing measurements. The method further includes deposition 1340 of a second metal layer on the dielectric layer and deposition 1350 of ferroelectric oxide layer on the second metal layer and deposition 1360 of the third metal layer on the ferroelectric oxide layer to form negative capacitor. The extended portion of the first metal layer and/or the third metal layer can be pattered to form a resistor. Also, in some implementations, the first and third metal serves as the input terminal and second and third mental serves as the output terminal.

(61) In some implementations, the method includes etching away the ferroelectric oxide layer from the extended region so that an electrical contact can be made with metal the first metal layer while doing measurement. The dielectric and ferroelectric oxide layers can be deposited using one or combination of an atomic layer deposition (ALD), a chemical vapor deposition (CVD), Metal-Organic Chemical Vapor Deposition (MOCVD), a Molecular Beam Epitaxy (MBE), a Metal Organic Vapor Phase Epitaxy (MOVPE), a Plasma Enhanced Chemical Vapor Deposition (PECVD), and a microwave plasma deposition.

(62) Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the objective of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.